In recent years, researches have been carried out to manufacture a semiconductor apparatus in which a group II oxide such as zinc oxide is used as a semiconductor layer. These researches, in which improvement of a semiconductor apparatus is their object or means, have reported a number of high-quality and concrete techniques. Broadly speaking, the techniques show that a performance of the semiconductor apparatus is effectively improved by improving crystallinity of a layered film, by controlling the crystallinity, by performing an interface control accompanied with them, or doping impurity. Based on that, the techniques teach, for achieving the effective improvement, modification in methods for layering films, film materials, and doping materials.
Among these reports, considerable techniques have been elaborated to concrete achievements as filing of patent applications. Here, four conventional techniques (References) are explained.
For example, described in Japanese Laid-Open Patent Application Tokukaisho No. 63-101740 (published on May 6, 1988; hereinafter, referred to as “Reference 1”) is a vertical electric field effect transistor gas sensor in which zinc oxide or the like is used as a channel layer.
Further, disclosed in Japanese Laid-Open Patent Application Tokukai No. 2000-150900 (published on May 30, 2000; hereinafter, referred to as “Reference 2”) is a technique for improving aperture ratio of a liquid crystal display apparatus by using a transparent transistor by using (i) a transparent semiconductor, made of zinc oxide or the like, for a channel layer of the transistor; and (ii) a transparent insulation oxide for a gate insulating layer, and the like means.
Further, a key point of Japanese Laid-Open Patent Application Tokukai No. 2000-277534 (published on Oct. 6, 2000; hereinafter, referred to as “Reference 3”) is that, instead of a sapphire substrate used in many researches, a substrate made of crystalline ScAlMgO4 or the like is used as the base film so that crystal lattice mismatching is eliminated between zinc oxide and the base film. Because of the use of the substrate made of crystalline ScAlMgO4 or the like, the crystal lattice mismatching is dramatically improved to be 0.13%. This technique is excellent because the Reference 3 describes that a layer of substantially single crystal zinc oxide can be obtained by layering zinc oxide on the substrate. In other words, the Reference 3 describes that crystallinity improvement of zinc oxide is a key for performance improvement of a semiconductor apparatus. Therefore, in order to improve crystallinity of zinc oxide, the substrate that is made of ScAlMgO4 or the like with some consideration with respect to the crystal lattice mismatching is used as the base film. With this arrangement, the crystallinity of zinc oxide is dramatically improved.
Disclosed in Japanese Laid-Open Patent Application Tokukai 2002-76356 (published on Mar. 15, 2002; hereinafter, referred to as “Reference 4”) is a method for doping 3d transition metal element to zinc oxide so that ON-OFF ratio property and a mobility property of a transistor, which includes a transparent channel layer made of zinc oxide or the like, will be improved.
In a vertical type transistor of a specific example of Reference 1, aluminum oxide is used as a substrate, and zinc oxide is layered on the substrate. It is assumed that the arrangement is made, in consideration of crystal lattice matching, for the purpose of crystallinity improvement of zinc oxide.
Further, in Reference 3, because the crystal lattice matching is taken into consideration, there is a premise that zinc oxide, ScAlMgO4, and the like are crystals.
These are because, as described above, there is a premise that a performance of a semiconductor apparatus is effectively improved by improving crystallinity of a layered film, controlling the crystallinity, and performing an interface control accompanied with them.
Further, in specific examples of References 2 and 4, zinc oxide is used as a channel layer, and a gate insulating layer made of aluminum oxide, ScAlMgO4, or the like is formed on the channel layer. In this way, the transparent transistor is prepared.
References 2 and 4 describe that material of the gate insulating film is transparent, and exemplifies a plurality of the transparent materials. Aluminum oxide is one of them. However, the aluminum oxide is exemplified not in terms of the interface control between the gate insulating film and the channel layer, but exemplified as a general material of a transparent insulating film. By way of exception, ScAlMgO4 is described as a transparent insulating material for the crystal lattice matching. Therefore, in this case, there is a premise that the transparent material is crystal.
However, in the case where a crystal material is used for the material as described in References, there are following restrictions 1 through 3:    1. It is difficult to form a film.    2. It is difficult to use a large substrate.    3. Order of forming films is fixed.
The restriction 1 is specifically explained as follows. For forming a predetermined crystalline film of zinc oxide or the like, determination of a precision apparatus and a precision condition, and secure maintenance of them are indispensable, as in the pulse laser deposition method or the like. Therefore, the techniques need to be more modification for mass production. Note that the same is true in the film formation of the substrate made of ScAlMgO4.
As for the restriction 2, although determination of a precision apparatus and a precision condition, and the secure maintenance of them are indispensable for forming the film made of predetermined crystal of zinc oxide, ScAlMgO4, or the like, it is difficult that a large substrate for a liquid crystal fulfills the requisites. Therefore, the techniques also need to be more improved for wider application to various types of products.
The restriction 3 is explained as follows. The formation of the film made of the predetermined crystalline of zinc oxide or the like, requires the formation of the substrate made of the predetermined crystal of ScAlMgO4 or the like as the base film, for the sake of the crystal lattice matching. Therefore, the base film of ScAlMgO4 inevitably has to be formed before the formation of the film of zinc oxide or the like. Therefore, for example, in the case of an FET, the techniques can be applied only to a staggered FET, in which a gate electrode is provided opposite to a substrate. This is not preferable because this narrows a freedom in designing a semiconductor apparatus.
Although the Reference 2 discloses the technique for improving performance of a semiconductor apparatus having a transistor in which zinc oxide is used, the Reference 2 does not discuss an improvement of the transistor itself, to simplification of manufacturing process, or the like.
Further, the Reference 4 discloses the technique for improving ON-OFF ratio property and mobility property of the transistor, however, the Reference 4 does not discuss the interface control, which affects a threshold shift and the like of electric properties of the transistor, particularly the interface control between the gate insulating film and the channel layer in the transistor.
As described above, the conventional techniques described in the References have high quality, however, have problems in their applications.